This invention relates to a method of preparing asymmetric semi-permeable membranes containing uniform microporous structure. The membranes are useful for gas or liquid separation and can also be used for preparing composite membranes.
Asymmetric membranes have been employed in separation of dissolved substances from solvents and vice versa, typically by the action of a pressure, including reverse osmosis, dialysis and ultrafiltration processes for the separation of gases and liquids. As is well known, an asymmetric membrane structure is characterized by graded porosity, that is, a graded pore size progressing from one major surface of the membrane to the other surface thereof. The structure may (depending upon the process by which the membrane is prepared) present the porosity thereof as graded interconnected pores either (a) ranging from a relatively coarse pore size at one membrane face to a smaller undetermined size adjacent a dense nonporous layer forming the second face or (b) ranging from a relatively coarse pore size at one membrane face to some very small finite value at the opposite face of the membrane through which gas can freely pass. Typically, the asymmetric membrane is composed of a dense thin layer and a spongy layer intended to act as a supporting layer.
Asymmetric membranes have been made by preparing a three-component coating solution containing a polymeric film forming material, a first good volatile solvent for the polymer and, relative to the first solvent, a poor or less-volatile solvent for the polymer. The good and poor solvents are mutually miscible. A membrane is cast from the solution at room temperature, allowed to desolvate for a short time and then immersed into a liquid precipitating agent which is miscible with the good and poor solvents but is a nonsolvent for the polymer, and then drying the membrane.
In yet another method, asymmetric membranes have been prepared by a process comprising dissolving a fluorinated polymer in a first solvent, optionally adding to the solution a second solvent having a vapor pressure different from that of the first solvent and spreading or casting the composition onto an appropriate substrate. The spread solution is allowed to evaporate for a short time and then immersed in an appropriate solution at different temperatures. Similarly, isotropic membranes have been prepared by a method comprising forming a casting dope of a polymer in an organic solvent, casting a film of said casting dope, preferentially contacting one side of the film with a diluent characterized by a high degree of miscibility with the organic solvent and a sufficiently low degree of compatibility with the casting dope to effect rapid precipitation of the polymer, and maintaining the diluent in contact with the membrane until substantially all of the solvent has been replaced with the diluent.
Another method for preparing porous films having asymmetric structural characteristics involves preparing a casting solution at room temperature consisting of a polycarbonate resin and a casting solvent composed of one or more good solvents, casting a layer of the casting solution onto a smooth surface or support, permitting desolvation to occur from said layer of casting solution, immersing the layer and support in a quenching bath liquid, the quenching bath liquid being capable of dissolving the casting solvent and causing swelling of the polycarbonate resin content of the layer while being a nonsolvent for the polycarbonate resin. The immersion step initiates formation of a microporous membrane by entry of the quenching bath liquid into the casting layer and the removal of the casting solvent from the layer. The microporous membrane is removed from the quenching bath and any remaining casting solvent and quenching bath liquid is removed from the membrane.
While the prior art methods have provided acceptable asymmetric membranes, there is a continuing need for asymmetric membranes which display a high efficiency for the separation of individual materials from their mixtures.